This invention relates to oxidation of aluminum-containing ferritic stainless steel foil under conditions that form oxide whiskers that substantially cover the foil surface and improve adhesion of an applied coating. More particularly, this invention relates a method for growing the whiskers that comprises heating the foil within a first temperature range to initiate the growth and thereafter within a relatively higher temperature range to continue the growth at an accelerated rate so as to reduce the time required to grow mature whiskers.
A metal monolith automotive catalytic converter typically comprises an aluminum-containing ferritic stainless steel foil substrate that carries an alumina coating impregnated with noble metal catalyst. Suitable iron-base alloys for the substrate comprise, by weight, about 15 to 25 percent chromium and about 3 to 6 percent aluminum. The alloy also may contain up to 1 percent yttrium or up to 0.1 percent rare earth metal, most notably cerium, to enhance high temperature corrosion resistance.
Prior to coating, the foil is oxidized to form a protective oxide layer. U.S. Pat. Nos. 4,331,631 and 4,318,828 describe oxidation treatment of the foil to form an oxide layer characterized by densely spaced whiskers that improve adhesion of the applied coating. The foil may be formed by cold rolling or by a metal peeling process. For cold rolled foil, the foil is pretreated by heating in a low-oxygen atmosphere to form a thin, precursor oxide film suitable for nucleating the whiskers. Although whiskers may be grown on peeled foil without a low-oxygen pretreatment, such pretreatment is nonetheless preferred to assure consistent growth.
To grow the whiskers, the pretreated foil is heated in a readily oxidizing atmosphere such as air for an extended time. In general, whiskers of preferred size and shape for bonding an applied coating are formed by heating at temperatures between 900.degree. C. and 950.degree. C. At higher temperatures, whisker growth tends to be inhibited, most likely by the formation of non-whisker alpha alumina. The particular uppermost whisker-growing temperature is sensitive to alloy composition. For example, a typical commercial cerium-containing alloy is found to be covered by whiskers after treatment at 960.degree. C., but to form few, scattered whiskers when heated at 980.degree. C. On the other hand, it is desired to reduce the treatment time to reduce the cost. Even within the preferred 900.degree. C. to 950.degree. C. growing range, treatment times of eight hours or more have heretofore been required to grow whiskers of a suitable size for tightly bonding the applied coating.
It is an object of this invention to provide an improved treatment for growing oxide whiskers on aluminum-containing ferritic stainless steel foil, which method employs a multi-temperature whisker-forming oxidation treatment to reduce the time required to grow mature whiskers.